What are the advantages of Rigify versus MakeHuman's MHX rig?
I tested out 2 options...

MakeHuman Game Rig Preset:


MakeHuman > Pose/Animate tab > Rig presets > select game.json
Then export to Blender.


Works like a charm in Blender. It seems to have all the posing that Rigify offers. Imported from Blender to Unity3D and worked perfectly with Mecanim. This seems to be the way to go!

MakeHuman with Rigify option checked on export:


No Rig preset set.
On export screen, select "Export for Rigify".


Works great in Blender. Requires that you filter for WGT (widgets) and delete them before importing into Unity3D. However, some of the deformations don't act right with Mecanim in Unity3D. According to Unity's Docs you will need to make adjustments to parents/skinning/etc in Blender before it works. 

Conclusion: MakeHuman's own rig preset wins, for a smoother blender-to-unity work flow, and there are no drawbacks in rigging comparision with Rigify that I noticed.